Conveyor belt systems are widely used to transport objects in various industrial, assembly and automation applications. For transporting relatively small objects, relatively small conveyor belt systems are used with dimensions on the order of several feet in the longitudinal (direction of conveyance) direction and two inches to several feet in the transverse direction.
A conventional conveyor belt system, shown in FIG. 1, includes a drive pulley 104, a tail pulley 108, a frame 106, a bed 106A and a conveyor belt 102. The belt 102 is looped around the pulleys 104, 108 and over the bed 106A. The drive pulley 104 is driven to rotate in the direction R1 by a driver (not shown) such as a conventional drive motor.
The rotating drive pulley 104 maintains rolling contact with the belt 102, thereby driving the belt 102 to rotate in the direction R2 around the frame 106. The tail pulley 108 also maintains rolling contact with the belt 102, and freely rotates in the direction R3 in response to the rotation of belt 102. In this way, the tail pulley 108 supports rotating belt 102 without significantly impeding its rotation.
Frame 106 supports the pulleys 104, 108 so that they are appropriately spaced apart from each other. Frame 106 also includes an integral bed 106A. The integral bed 106A is generally constructed as a unitary piece with the rest of the, frame 106, or is permanently fixed thereto.
The bed 106A provides a relatively slick and relatively continuous surface to support the underside of the belt 102. Because of the fairly continuous surface provided by the bed 106A, objects placed on the top of the belt 102 will be substantially continuously supported by the underlying bed 106A, thereby minimizing shear stress and strain on the belt 102 itself. Because the bed 106A is relatively slick, the belt 102 will slide over the bed 106A with relatively low friction, even when objects on top of the belt 102 weigh the belt 102 down onto the bed 106A, thereby minimizing longitudinal forces in the belt 102.
As shown in FIG. 1, the bed surface defines a line, herein called the bed height BH. The belt 102 travels over and along this bed height BH line. The tops of the drive pulley 104 and the tail pulley 108 are both co-linear with the bed height BH line. In other words the pulleys 104, 108 have an outer radius of H1 so that the tops of these pulleys reach the level of the bed. Thus, the height of the pulleys match the height of the bed.
This matching of pulley and bed heights is important for several reasons. First, if there is a disparity in heights between the pulley and the bed, then an object being transported on top of the belt 102 may be jolted as it travels over a portion of the system 100 where there is a transition in height between the bed 106A and a pulley 104 or 108. This kind of jolting caused by mismatched heights may be especially troublesome in application where two conveyor systems are placed end to end to effect a longer conveyor run.
Second, if bed 106A is significantly lower than the height of the pulleys, then the belt 102 will not be supported by the bed 106A. When heavy objects are placed on the belt 102, the belt 102 may be (temporarily or permanently) deformed by objects pushing the unsupported belt 102 down to the level of the bed 106A.
Third, if the bed 106A is significantly higher than the pulleys 104, 108, then the belt 102 will be pulled tightly around the transverse edges of the bed. This increases wear on the belt 102.
Fourth, if the bed 106A is significantly higher than the pulleys 104, 108, then the contact area between the belt 102 and the drive pulley 104 will be reduced, thereby decreasing the load which the drive pulley 104 can effectively drive the belt 102 to convey. For at least these reasons, matching pulley and bed height is an important precept in the design of most conveyor belt systems.
In the embodiment of FIG. 1, the heights of the pulleys 104, 108 and the bed 106A are exactly the same (all heights are at the BH line). However, depending on factors such as the material of the belt, optimal performance may involve making the height of the bed either slightly higher or slightly lower than the height of the pulleys. In other words, the height of the pulleys may be slightly displaced from the bed line BH.
For example, if a conveyor belt is made of a stiff material, then the belt may not follow the outer surface of each pulley for a full 180.degree. (even with an appropriate degree of tightening), and the belt may therefore come off the pulley at an angle relative to the tangent direction taken at the top of the pulley. This phenomenon is known as cupping. In this case, the frame may optimally be designed so that the bed is a bit higher than the top of the pulley, to appropriately account for the angle at which the belt comes off of the pulleys.
As used herein, the pulleys and bed are "matched" in height when the height of the pulleys and the height of the bed are close enough to each other to provide good performance and a low degree of belt stress, strain and wear, especially in view of the above-described problems caused by wide height disparities. As used herein, the pulleys and bed may be "matched" in height, even if their heights are not exactly the same, whether the slight disparity in heights is a result of design or random variations (such as manufacturing variations).
It is also noted that two pulleys and a bed may be matched in height even if the pulleys have different radii. In order to be matched in height, the top of each pulley should merely be sufficiently close to the height of the bed for optimal performance under the circumstances of the application.
Another embodiment of a conventional conveyor belt system 200 is shown in FIG. 2. Conveyor belt system 200 includes a belt 202, a drive pulley 204, a frame 206, a tail pulley 208 and a bed 210. The conveyor belt system 200 is similar to conveyor belt system 100, except that instead of an integral bed such as 106A, the bed 210 is connected to frame 206.
One advantage of such a detachable bed 210, is that the bed 210 can easily be made from a different material than the frame 206. For example the frame 206 may be made from metal, while the bed 210 might be made of ultra high molecular weight polymer (herein UHMW), which provides a smooth, slick supporting surface for the belt 202. However, conveyor system 200 cannot be used without the detachable bed 210 for two reasons explained below.
First, if the system 200 is used without the detachable bed 210, then the pulley height and the height of the frame 206 (without a bed) will be drastically mismatched. The pulleys 204, 208 have an outer radius of H4 and a resulting height of BH'. Likewise, the bed 210 also has a height of BH'. More specifically, as shown in FIG. 2, when the detachable bed 210 is in place, the aggregate height of the frame 206 (H2) and bed 210 (H3) adds up to H4, thereby matching the height of the pulleys 204, 208 at the bed height BH' line. If the bed 210 is removed, ther the height of the frame H2 would fall short of the BH' line defined by the tops of the pulleys 204, 208, and the heights would be problematically mismatched.
Second, the frame 206 (without the detachable bed 210) does not provide a good bed surface for the belt 202, because it is not continuous. FIG. 3 shows the frame 206 from its underside. The frame 206 is actually a lattice of several elongated, aluminum members 212, 214, 216, 217, 218, 220, 222.
More specifically, the frame 206 is assembled from two extruded side walls 212, 214, three transverse members 216, 217, 218 and two support members 220, 222. While this frame 206 is considerable lighter and easier to fabricate than a solid aluminum frame would be, the frame does not provide a continuous surface appropriate for supporting a load bearing conveyor belt (as shown in FIGS. 3 arid 4). This makes bed 210 a necessary component of conveyor belt system 200.
Because the conveyor belt system 200 requires bed 210, the pulleys 204, 208 must be chosen so that the top of each pulley 204, 208 corresponds with the aggregate height of the frame and bed assembly. In both conventional conveyor belt systems 100, 200 described above, the effective height of the bed must be determined when the system is designed so that the height of the pulleys will match the height of the bed (integral 106A or detachable 210) which will be used.
As will be understood, conventional conveyor belt systems such as 100, 200 do not allow for any modifications which would change the effective bed height, because a change in bed height would necessitate a change in the pulleys, which is an extremely difficult change to make in practice. For example, a change to larger pulleys can cause physical interference between the pulleys and the frame. Therefore, any modification in bed height will generally require an entirely new conveyor system, essentially designed from scratch, so that the pulley height appropriately matches the bed height.